His work was interrupted in October, 1943 when he and other university staff members were arrested by the Nasjonal Samling and handed over to the occupation authorities. He spent time in several detention camps, until he was released in November, 1944.[4]

Hassel originally focused on inorganic chemistry, but beginning in 1930 his work concentrated on problems connected with molecular structure, particularly the structure of cyclohexane and its derivatives. He introduced the Norwegian scientific community to the concepts of the electric dipole moments and electron diffraction. The work for which he is best known established the three-dimensionality of molecular geometry. He focused his research on ring-shaped carbonmolecules, which he suspected filled three dimensions instead of two, the common belief of the time. By using the number of bonds between the carbon and hydrogen atoms, Hassel demonstrated the impossibility of the molecules existing on only one plane. This discovery led to his being awarded the Nobel Prize in Chemistry for 1969.